Ah, I've heard back from the agent. She read Stillwater all the way through, she gave it to the head of her agency and discussed it with her colleagues; she likes the main character and the setting; she thinks the moral questions work.
But ultimately what [agency head] and I both felt after reading was that this doesn't quite transform the book in a new way enough. I really enjoyed reading it -- but it felt like a beat-for-beat retelling of Mansfield Park to me. I never quite felt like the characters took on their own life or surprised me. They felt real, and I cared about them, but I didn't feel like I had learned something new about Mansfield Park by the end of this book. I wanted a little bit more from Melly, somehow -- not anything different about her moral sensibility, but because she's such a great character and you're obviously so passionate about her, I wanted some deeper realization from her. I also have to admit that Malcolm doesn't quite make the transition to modern life well -- I was really angry at him for his dumb infatuation with Alys, as I think I was supposed to be, but it was a little hard to understand how we switched to loving Melly so quickly. I think their relationship needs a little more development, somehow. (This may be part of figuring out whatever it is that's missing from Melly's arc too.)
I'm sorry not to have better news for you! Possibly another agent will feel differently, but I wanted to share our feedback in case it struck a chord with you, because I do wonder if some further revising/rethinking would make this book something really spectacular. There's so much that's working that I suspect it could be a real hit, but it feels like it's only 90% of the way there.
...I would just look for agents open to upmarket women's fiction or literary fiction and query widely. You might also consider Catholic presses if there are some reasonable ones, given the strong Catholic angle of the book.She thinks I'm a beautiful writer and offers to read my next book, if I don't find an agent for Stillwater.
I'm taking it as a favorable sign that she read it all the way through and shared it, and that she offers a quantity of feedback. All in all, it's not a surprise that the first agent I sent the book to won't take it. But how convenient it would have been to have it all fall into my lap like that, without having to put in the hard work of querying...